Semiconductor memory is widely used in various electronic devices such as cellular telephones, digital cameras, personal digital assistants, medical electronics, mobile computing devices, and non-mobile computing devices. Semiconductor memory may comprise non-volatile memory or volatile memory. A non-volatile memory allows information to be stored and retained even when the non-volatile memory is not connected to a source of power (e.g., a battery). Examples of non-volatile memory include flash memory (e.g., NAND-type and NOR-type flash memory) and Electrically Erasable Programmable Read-Only Memory (EEPROM).
In flash memory and EEPROM devices, a memory cell can include a floating gate that is positioned above and insulated from a channel region in a semiconductor substrate, in a two-dimensional (2D) NAND configuration. The floating gate is positioned between source and drain regions. A control gate is provided over and insulated from the floating gate. The threshold voltage (Vth) of the transistor thus formed is controlled by the amount of charge that is retained on the floating gate. That is, the minimum amount of voltage that must be applied to the control gate before the transistor is turned ON to permit conduction between its source and drain is controlled by the level of charge on the floating gate.
The ability to adjust the threshold voltage allows a floating-gate transistor to act as a data storage element or memory cell. In some cases, more than one data bit per memory cell (e.g., a multi-level or multi-state memory cell) may be provided by programming and reading multiple threshold voltages or threshold voltage ranges.
NAND flash memory structures typically arrange multiple floating-gate transistors in series with and between two select gates. The floating-gate transistors in series and the select gates are referred to as a NAND string. In addition, ultra-high density storage devices have been proposed using a three-dimensional (3D) stacked memory structure which is formed from an array of alternating conductive and dielectric layers. One example is the Bit Cost Scalable (BiCS) architecture. A memory hole is drilled in the layers, and a NAND string is formed by filling the memory hole with appropriate materials. A straight NAND string extends in one memory hole, while a pipe- or U-shaped NAND string (P-BiCS) includes a pair of vertical columns of memory cells which extend in two memory holes and which are joined by a bottom back gate. Control gates of the memory cells are provided by the conductive layers.
In recent years, NAND flash memory has been scaled to reduce cost per bit. However, as process geometries shrink, many design and process challenges are presented. These challenges include widening of Vth distributions and increased program disturb.